1. Field of the Present Disclosure
The present disclosure relates to an image forming apparatus.
2. Description of the Related Art
A measurement method of a toner density on an image carrier using a reflection type optical sensor calculates an index (e.g. coverage factor mentioned below or the like) that indicates a toner density on the basis of a change of an output voltage of the reflection type optical sensor.
Such reflection type sensor is of a specular-reflection-and-diffuse-reflection-separating type or of a polarization splitting type.
Of the specular-reflection-and-diffuse-reflection-separating type, the reflection type sensor includes two photodetectors that receive specular reflection light and diffuse reflection light, respectively. Specifically, the specular-reflection photodetector is arranged on an optical axis of reflection light of incoming light, and the diffuse-reflection photodetector is arranged out of the optical axis. Outputs of these photodetectors are used for the detection of the toner density.
The polarization splitting type utilizes a polarization characteristic of color toner, and arranges a beam splitter, causes a specific polarized light to enter the beam splitter, splits the reflection light into P-polarized light and S-polarized light using the beam splitter, and receives the P-polarized light and the S-polarized light using two photo detectors. Outputs of these photodetectors are used for the detection of the toner density.
The detection of the toner density is performed on the basis of a ratio between a sensor output of a surface material part of the image carrier (i.e. a surface part on which toner does not adhere) and a sensor output of a toner part (i.e. a surface part on which toner adheres). Using this ratio gives an advantage to enable to exclude influence of dirt on a head part of a light emitting unit in an optical sensor, light intensity fluctuation of an LED (Light Emitting Diode) as a light emitter of an optical sensor and the like.
Under a condition that all incoming light to black toner is absorbed by the black toner and incoming light to color toner diffusely reflects completely, regardless of toner type (i.e. black toner or color toner), a coverage factor M of toner on an image carrier is expressed as the following formula.M=1−{(P−Pd)−(S−Sd)}/{(Pg−Pd)−(Sg−Sd)}
Here Pd is a dark potential of the specular-reflection light (P-polarized light) photodetector, Sd is a dark potential of the diffuse-reflection light (S-polarized light) photodetector, Pg is a P-polarized light component from the surface material of the image carrier, Sg is an S-polarized light component from the surface material of the image carrier, P is a P-polarized light component from the toner part, and S is an S-polarized light component from the toner part.
Even if actual toner densities of toner patterns on the image carrier are identical to each other, the coverage factors M (i.e. measured toner densities) of the toner patterns may be different from each other.
An image forming apparatus uses a multi-layer rubber transfer belt including an elastic layer as an image carrier on which a toner patter is measured by an optical sensor; and external additive of toner (i.e. abrasive that polishes a photoconductor) adheres on a surface of such transfer belt and thereby surface nature of the transfer belt may vary. It is proposed that in such a case, the endurance X (X=A×{1−(Sg−Sd)/(Pg−Pd)}, A: constant) is calculated from a sensor output, and the coverage M is corrected on the basis of the endurance X.
In general, a substance such as toner adheres on a transfer belt and thereby Sg, i.e. (Sg−Sd) increases due to a polarization characteristic of the adhering substance; and contrarily, Pg, i.e. (Pg−Pd) increases due to polishing the image carrier through use. Therefore, regarding a transfer belt originally having a high surface glossiness and a used transfer belt having a high surface glossiness due to toner adhering and polishing, it is supposed that even if (Pg−Pd) of the both transfer belt are equal to each other, (Sg−Sd) are different from each other, and consequently the endurances X are different from each other.
Further, when the glossiness of the belt surface material is high, since direct reflection light is much from the belt surface, Pg is high. Therefore, when the glossiness of the belt surface material is high, the term {(Pg−Pd)−(Sg−Sd)} is high in the calculation formula of the coverage factor M.
On the other hand, in a high toner density range, the influence of the belt surface material is small, and therefore, a value of the term {(P−Pd)−(S−Sd)} does not vary widely.
Consequently, the coverage factor M of the transfer belt is calculated as higher value due to a higher glossiness of the transfer belt
Meanwhile, as shown in FIG. 9, even if the glossinesses of the transfer belts are different from each other, the aforementioned endurances X may be substantially identical to each other. FIG. 9 shows a diagram that indicates an example of a relationship between a glossiness (a measurement value by a glossmeter) and an endurance X at plural conditions of a transfer belt.
For example, the endurance X at an initial state of a low glossy transfer belt and the endurance X of a used high glossy transfer belt can be substantially identical to each other.
In a case that the endurances X are identical to each other but the glossinesses are different from each other, even if the coverage factor M is corrected on the basis of the endurance X, the correction is not properly performed in consideration of the glossiness, and consequently, the coverage factor M varies in accordance with the glossiness.
It should be noted that it is possible to set a glossmeter, measure a glossiness of a transfer belt using the glossmeter, and correct a coverage factor on the basis of the obtained glossiness, but in a such case, setting the glossmeter causes a high cost of the apparatus.